Download Colorful Liquid Crys tal Fil ter Abstract Color filters are very important

Colorful Liquid Crys tal Fil ter Abstract Color filters are very important components for optical
equipment. Basically,color filters are made of high-density glass or isinglass. They can select a
certain part of aspectrum before further processed.However,the transmitting spectral band is
usually fixed. In Transmitting spectral band is usually fixed. In traditional color filtera. A
different filter has to be used if one would like to select a different spectral band. This way is
some what Inconvenient. In this study, we design and develop an adjustable color filter by
making use of the optical activity dispersion(OAD)in quartz and birefringence of the liquid
crystal (LC) under the in flu ence of ap plied electric field. The LC filter which is consisted by
one OAD plate,one LC shutter,and two polarizers is set up.The theoretical numerical calculation
using Jones maxtrix is per formed based on this configuration for guidance,and the
char ac ter iza tion experiments of the LC filter are also carried out . The results verify that an
spectral band tunable filter could be developed by this method. The intensity dis tri bu tion of
the transmission spectrum world then possess the cos pattern. Furthermore,if multiple passes are
employed,the spectral distribution will then be,with n being the total number of passes. Finally,an
electrid field tunable color filter is extremely useful for the application on optical process. 2
1.Motivation Color filters are useful and important com po nents in a lot of applications, such as
imaging de vic es and optical instruments. They are used to se lect the desired portion of a
spectrum from the in ci dent beam before further processing. However, the trans mit ting
bandwidth of them are often fixed in con ven tion al color filters. A different filter has to be
used to change the transmitting spectral band. Thus, it is of great interest to have a tun able color
filter. Optical activity was discovered in 1811 by Biot . The measurement of it is often used to
de ter 1 mine the concentration of optically active ma te ri als such as glucose. In this study, we
propose to make use of the dispersion of optical activity in quartz and the tuning capability of
liquid crystal display (LCD) devices to develop a tunable color filter.2.Purposes (1) In this
investigation, we want to explore the feasibility in using a LCD plate to rotate the po lar iza tion
plane of the incoming light. 2 3
(2)We then develop a tunable bandpass color fil ter that can be adjusted by changing the ap plied voltage on the LCD plate
3.Equipments A.Two polymer polarizers B.One liquid crystal display plate C.One quartz plate (Its surface cut perpendicula
optic axis as shown in Fig.1) D.One spectrometer (Grating & CCD) E.One Programmable Voltage Source F.Two mirrors(M
G.halogen light source (S) 4.Theory and Computation Algorithm The basic operational principle of the optical set up (Fig.
transmission coefficient can be de scribed by Jones Matrix as following: 2 1
Fig.2 Schematics of a tunable color filter setup.
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Fig.1 Optic axis in
The LCD plate functions as a voltage con trol la ble wave plate. And
ro ta tion al an gle of the polarization plane on LCD plate as a func tion of applied voltage. ,where d is the thick ness o
quartz plate, λis the wave length of the incoming beam. For neg a tive ly ac tive quartz, nR-nL is a constant 6.65
number of round-trips. The results of Jones matrix transformation can also be expressed by the following equation: or whic
in numerical simulation.5.Methods (1) Numerical simulation is performed according to the theory described in previous sec
and the flow chart of the numerical calcula-tion is shownin Fig.3.(2) The schematic diagram of the setup is shown in Fig.4(
transmitting spectra at various voltages were measured by a spectrometer and pho to graphs of the setup is displayed in Fi
p8.
-5
Flowchart of numerical simulation.
would decreases. In order to compare, the peak
height has been normalized. In Fig.6(a),when we
increase the thickness of quartz plate, the rotatory
dispersion will be mul ti plied so that we can get
a narrower peak in trans mit ting spectrum(see
Fig.6(b)), but at the same time, second order
peaks also appear in the spectrum. In Fig.7
(a),when we adjust the voltage, the peak of the
transmitting spectrum will move gradually. The
bandwidth of the peak is linear to the wave length
Fig.3 of it.(see Fig.7(b)).
Fig.4(a) Schematic diagram of experimental apparatus.
6.ResultsA.Simulation Hypothesis: under proper voltage applied to LCD plate, the polarization
plane of the ray pass ing through the LCD plate can be rotated from 0°to 90°. In Fig.5, we can
easily find out the bandwidth (full width at half maximum ;FWHM ;Γ) of the transmission
spectrum is narrowed down while the ray passes the filter more than one time. But as the round-
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trip increase, the ef fi cien cy of multi-pass
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B. Experiments In Fig.8 and Fig.9, we can see
clearly how the peak move whileφr(V) Changes.
In terms of bandwidth, three-pass is much
narrower than sin gle-pass, which agrees well
with numerical sim u la tion (Fig.5). Fig.10
demonstrates the transmitting spectra of ray
passing through the colorful liquid crystal filter at
dif fer ent voltage. As applied voltage increases,
the peak wavelength moves from red (620nm) to
green(520nm) and the bandwidth is also narrowed
down. Advanced analysis of Fig.10 is shown in
Fig.11 & Fig.12. The peak wavelength(λp) is
linear to applied voltage(Va). This implies that we
can ad just the transmitting band of the filter
lin ear ly by voltage control (Fig. 11). On the
other hand, the bandwidth becomes nar row er at
shorter peak wavelength as shown in Fig.12.
Fig.4(b)
Photo of the setup.
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7.Discussion 1. From experimental measurements, we
have found that the bandwidth(λ) of the transmitting spec trum is linear to the wavelength
(Fig.12), which agrees well with the results from numerical simulation(Fig. 7). Therefore, the filter
would have better per for mance in the blue part of the spectrum. 2. The transmission of the filter
decreases as the peak wavelength decreases, which can be ac count ed for the decreasing
transmission of poly mer polarizers at shorter wavelength. 3. These filters can provide immediate
ap pli ca tions such as color light control in stage or var i ous il lu mi na tion setups. 4.
Materials with high optical activity dispersion would further facilitate the developments of such
color filters and extend the wavelength range of application to UV and IR. 5. Furthermore, we can
make use of Fabry-Perot interference to narrow the bandwidth of trans mit ting
spectrum.8.Conclusion In this work, we have successfully simulated and designed as well as
developed a tunable color fil ter based on the dispersion of optical activity. The transmitting
spectral band can be tuned by ad just ing the applied voltage on the LCD plate. Nar row er
transmitting band can be im proved through multi-pass setup and dis per sion control by
adjusting the thickness of the quartz plate. This new colorful fil ter offers a device to the
ap pli ca tion of the electric con trolled color light. 4
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9.References 1. E. Hecht, Optics, pp309-314,
Addison-Wesley Pub. Comp., MA,1990 2. Y.
P. Chiao, Liquid Crystal & Liquid Crystal
Display,
http://www.ep.nctu.edu.tw/teachercamp/
water%20con.htm, 1999 3. A. Yariv and P.
Yeh, Optical Waves in Crystals, pp. 94-103,
John Wiley, New York, 1984 4. Floch,
Selective Optical Resonator, p1, US. Patent,
1984
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